Efficient high harmonics generation (HHG) was demonstrated at 10 MHz repetition rate with an external femtosecond enhancement cavity, seeded by a ${\sim}70~\text{fs}$ post-compressed 10 MHz fiber chirped pulse amplifier (FCPA) laser. Operation lasting over 30 min with 0.1 mW outcoupled power at 149 nm was demonstrated. It was found that shorter pulse was beneficial for alleviating the nonlinear plasma effect and improving the efficiency of HHG. Low finesse cavity can relax the plasma nonlinearity clamped intra-cavity power and improve the cavity-locking stability. The pulse duration is expected to be below 100 fs for both 1040 nm and 149 nm outputs, making it ideal for applications such as time-resolved photoemission spectroscopy.

The general issue of the spectral selection of a portion of the wide extreme-ultraviolet spectrum obtained via extreme nonlinear processes as high order harmonic generation includes the problem of maintaining the ultrafast temporal duration of the pulses. In this paper, we present an instrument in which the pulse selection is operated in the wide wavelength range from 17 nm to above 60 nm, which is the central portion of the high-harmonics spectrum, with an instrumental function of about three femtoseconds. The design of the monochromator is based on the conical diffraction, which realizes very high diffraction efficiency by exploiting the specular reflection on the grating facets long-wise illuminated. The optical layout makes use of two gratings in the compensated-monochromator scheme already presented by the authors. The discussion of the residual aberration is also presented, with the aims to investigate the ultimate temporal resolution obtainable by this scheme.

In this article, we demonstrate the generation of four phase-locked harmonic pulses separated in time using frequency-domain interferometry. The spectra present a high sensitivity to a change of the relative phase on an attosecond time scale. The spectral resolution and the control of this relative phase could be used to perform high resolution measurements.